Heat a pan and add the oil. Tip in the onion, potatoes and chillies and mix. Stir occasionally for 3 minutes. Fold in the tomato puree. Add the cumin, turmeric and salt and mix again. Tip in 300ml of just boiled water. Cover and simmer for 10 minutes or until the potatoes are cooked. Serve hot with pitta breads, chapatis or basmati rice.